1. Field of the Invention
This invention relates to a beam converting optical system. More particular, this invention relates to a beam converting optical system that converts light beams from a semiconductor laser having an oval cross sectional shape to form parallel light beams having a substantially circular cross-sectional shape.
2. Description of Related Art
Typically, semiconductor lasers are used as light sources in optical disk heads. Since the light emitting surface of the semiconductor laser is oval-shaped, the intensity distribution of the emitted light beams are also oval in cross-sectional shape. In order to efficiently use the emitted oval cross-sectional shaped light beams, the light beams are converted from an oval cross-sectional shape to a substantially circular cross-sectional shape.
FIG. 7 shows the structure of a conventional beam converting optical system that uses a beam converting prism 52. A light beam from the semiconductor laser LD is converted by the collimator lens 51 into parallel light beams. The parallel light beams strike the beam converting prism 52 and are expanded in the up and down direction to become parallel light beams having a substantially circular cross-sectional shape. The converted parallel light beams pass through the beam splitter 53, the object lens 54 and are collected on a disk 55. The light reflected from the disk 55 passes back through the object lens 54 and is deflected in the upward direction by the beam splitter 53 to undergo signal detection.
The prism 52 is a converting optical element having a simple structure. The light path of the semiconductor laser LD with the collimator lens 51 and the light path of the beam splitter 53 with the object lens 54 are at different angles with respect to the prism 52. Therefore, the positioning of each optical element becomes difficult in conventional beam converting optical systems.
In addition, the laser light strikes the converting prism 52 at an angle of incidence .theta. of approximately 70.degree.-80.degree.. In order to suppress the reflection of the laser light at the incident surface of the converting prism 52, it is necessary to provide a reflection preventing membrane consisting of a multilayer coating on the incident surface of the converting prism 52. However, this reflection preventing membrane involves a substantially higher cost.
FIGS. 8(a) and 8(b) show the structure of conventional beam converting optical systems that use a cylindrical lens rather than a prism. In particular, FIG. 8(a) shows a Galilean type converting optical system while FIG. 8(b) shows a Keplerian type converting optical system.
In the Galilean type converting optical system, a light beam from the semiconductor laser LD is converted into parallel light beams by the collimator lens 61. The light then strikes a cylindrical lens 62 having a negative refractive power in the Y-direction. The cylindrical lens 62 does not have any refractive power in the Z-direction including the optical axis. After the light has been suitably expanded by the cylindrical lens 62 in the up and down directions the light passes through a collimator lens 63 having a positive refractive power in the Y-direction while generally not having any refractive power in the Z-direction including the optical axis. The light passing through the cylindrical lens 63 that has been converted into parallel light beams and has a substantially circular cross-sectional shape then passes through the beam splitter 64, the object lens 65 and is collected onto the disk 66.
In the Keplerian type converting optical systems a light beam from the semiconductor laser LD is converted into parallel light beams by the collimator lens 63. The light that has been converted into parallel light beams strikes the cylindrical lens 67 having a positive refractive power in the plane of the figure without having any refractive power in the plane perpendicular to the figure including the optical axis. After the light strikes the cylindrical lens 67 and is expanded in the up and down direction, the light strikes a collimator lens 63 having a positive refractive power in the plane of the drawing and not having any refractive power in the plane perpendicular to the figure including the optical axis The light passing through the collimator lens 63 is converted into parallel light beams having a substantial circular cross-sectional shape.
For both the Galilean type shown in FIG. 8(a) and the Keplerian type shown in FIG. 8(b) it is necessary to reduce the NA (numerical aperture) so as to suppress aberrations. That is, it is necessary to increase the spacing between the two cylindrical lenses. This increases the overall length of the optical system.
As described above, when using conventional beam converting optical systems having beam converting prisms, it is very difficult to position each optical element since the optical axis is bent by the prism. In additions since the incident light beam is at a large angle of incidence with respect to the beam converting prism, it is necessary to provide a reflection preventing membrane on the incident surface of the beam converting prism that results in an increased cost. Additionally, when using conventional beam converting prisms that use cylindrical lenses, it is necessary to make the overall length of the optical system inconveniently large in order to suppress aberrations.